1. Technical Field
The present application relates generally to electromagnetic logging of subsurface formations surrounding a wellbore using data obtained with a logging tool, and particularly to using coupling measurements obtained with an electromagnetic tool used to log while drilling.
2. Background Art
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements to infer properties of the formations surrounding the borehole and the fluids in the formations. Common logging tools include electromagnetic tools, nuclear tools, and nuclear magnetic resonance (NMR) tools, though various other tool types are also used. Electromagnetic logging tools typically measure the resistivity (or its reciprocal, conductivity) of a formation. Prior art electromagnetic resistivity tools include galvanic tools, induction tools, and propagation tools. With propagation tools, typically a measurement of the attenuation and phase shift of an electromagnetic signal that has passed through the formation is used to determine the resistivity. The resistivity may be that of the virgin formation, the resistivity of what is known as the invasion zone, or it may be the resistivity of the wellbore fluid. In anisotropic formations, the resistivity may be further resolved into components commonly referred to as the vertical resistivity and the horizontal resistivity.
Early logging tools, including electromagnetic logging tools, were run into a wellbore on a wireline cable, after the wellbore had been drilled. Modern versions of such wireline tools are still used extensively. However, the need for information while drilling the borehole gave rise to measurement-while-drilling (MWD) tools and logging-while-drilling (LWD) tools. MWD tools typically provide drilling parameter information such as weight on the bit, torque, temperature, pressure, direction, and inclination. LWD tools typically provide formation evaluation measurements such as resistivity, porosity, and NMR distributions (e.g., T1 and T2). MWD and LWD tools often have characteristics common to wireline tools (e.g., transmitting and receiving antennas), but MWD and LWD tools must be constructed to not only endure but to operate in the harsh environment of drilling.
Traditional electromagnetic (EM) measurements assume simple formation models to obtain quantitative measurements. For example, classical compensated resistivity assumes a vertical well and a homogeneous formation. In this case, the measured EM coupling can be related to an “apparent resistivity”, that being the resistivity that would produce the same measured coupling. This relation or, more particularly, this transform assumes the formation to be homogeneous. If that assumption is not the true, then the apparent reading must be corrected (e.g., shoulder bed correction, invasion correction, etc.).
Recently, deep directional EM tools have been implemented that measure the off-diagonal terms of the EM or coupling tensor. Based on those measurements, new quantities have been derived, called “Symmetrized Directional” (SD) measurements or responses, that indicate the presence of a resistivity contrast. This measurement applies well to cases in which the formation comprises parallel layers of contrasting resistivities. It is possible to use those measurements to compute the distance to a boundary of contrasting resistivity and the change in conductivity. However, from just this information, it is impossible to determine if the formation has parallel boundaries between layers or if its geometry is more complicated.
If a tool has enough coils (i.e., antennas) with different orientations at two points, it is possible to measure the whole set of EM-couplings between those two points. However, interpreting those raw couplings can be very tedious, particularly with complicated geometries. Interpretation is currently practical for cases in which the formation is assumed to have parallel layers (1-D formation). In vertical wells, classical compensated resistivities are useful, and in horizontal wells, Symmetrized Directional measurements are good indicators of distance to boundaries. So, in conjunction with bulk resistivity measurements, it is possible to estimate a formation resistivity distribution. However, this technique does not apply if the formation is not 1-D.